It has been disclosed in U.S. Pat. No. 5,488,082, that adhesive pastes for attaching silicon dies onto ceramic substrates can be prepared from at least one organic polymer resin, inorganic filler, and a fugitive liquid, in which the resin and the filler are present in particulate form of small enough size to pass through a number 325 mesh screen, and in which the fugitive liquid and organic polymer resin are substantially insoluble in each other.
The polymer is preferably a thermoplastic polymer, but may be a combination of a thermoplastic polymer and a thermoset polymer, provided that the thermoplastic nature of the blend is preserved. The maximum particle size for the organic polymer resin is that particle size that can pass through a 325 mesh screen, and preferably through a 400 mesh screen.
These paste compositions are used for attaching semi-conductor chips or dies to a substrate. The paste is applied to a surface of the substrate to form a bond line and the semi-conductor component is placed on the bond line so that the paste is between the die and the substrate. This electronic assembly is then heated to a sufficiently high temperature for a sufficient time so that the thermoplastic resin softens and becomes fluid and the liquid evaporates from the paste. The assembly is cooled to a temperature below which the thermoplastic polymer becomes solid resulting in the bonding of the microcircuit to the substrate. The advantage of using this adhesive paste is that the bond line is void free. Voids in the bond line eventually can result in adhesion failure.
These pastes were tested as die attach compositions using 8.6 mm.times.8.6 mm (350 mil.times.350 mil) silicon dies (semi-conductor dies) on a ceramic substrate. However, these compositions have been shown to have limited utility on metal substrates or on larger die sizes, particularly for semi-conductor dies of sizes 400 mil.times.400 mil or greater. When these prior art adhesive pastes are used on dies of this size, and particularly on large size dies bonded to metal substrates, the dies delaminate or separate from the substrate after the curing cycle for the adhesive. The large die size makes it more difficult for the fugitive liquid to escape, which results in voids. Also, the difference between the coefficient of thermal expansion (CTE) of a metal substrate and that of the die is large. This mismatch of CTE places the adhesive bond between the die and substrate in constant stress and can result in die warpage and delamination. This combination of factors, large die size and metal substrate, almost inevitably lead to delamination of the die from the substrate.
Delamination is a critical problem, especially for those circuits that have high power, and consequently, high heat generating applications. In some instances, even if there is some delamination, the microcircuitry silicon chip or die will remain successfully bonded to the substrate. However, when there is a high heat output, even a minor amount of delamination from the substrate will detract from the ability of the adhesive to transfer the heat generated in the circuitry into the substrate. Eventually, this could result in device failure. Thus, there is still a need for improved adhesive compositions for use in attaching semi-conductor silicon dies, particularly large dies, to substrates, and particularly to metal substrates.